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| A
study of MADAGASCAR AMETHYST |
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| ( SiO2
) Faceted Amethyst
click here ! |
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| (Variety
of Quartz) |
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| ·
Crystallographic
properties: |
| Trigonal
System (Subdivision of the hexagonal system) |
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| Crystal structure of amethyst
: |
| Quartz
forms hexagonal prisms at the blunted ends and the head |
| finishing
in hexagonal pyramids. |
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| ·
Physical
properties |
| Hardness
7 |
| Density
2.65 - 2.66 |
| Refractive Index: 1.54 -1.55 + 0.009 positive
uniaxial |
| Glare:
vitreous |
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| ·
Chemical Composition: |
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| Silicium Dioxide (
SiO2 ) |
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| The
colouring of amethysts is due to the presence of colour centers which come
from substitution of ions of |
| silicon
by iron ions in the crystal lattice of quartz. |
| For
example, the
amethyst crystallizes at lower temperatures than smoky quartz.In
Madagascar, we find it either in |
| crypts of pegmatites, or in quartzite veins in connection with those. |
| The
geodes of the siliceous nodules of basalts contain some too. |
| The
first have an hexagonal network, although their pattern of crystallization
is only of ternary order. The
seconds |
| have
a ternary network. The
elementary mesh is a rhomboedron, i.e. a parallelepiped consisted six
equal |
| rhombuses. |
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| A
ternary axis A3
joint the tops of the regular trihedrons, three normal binary axes A2
with the ternary axis joining |
| the
meddle of opposite horizontal corners. |
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| Here
elements of symmetry of the Trigonal system with |
| the
oblique shape, the tetragonal scalenoedron. |
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| It
is interesting to see what becomes this tetragonal scalenoedron in
others classes where the disappearance of |
| the
symmetry planes makes decrease of half the number of the faces. We
obtain a trapezohedron (One notices that |
| these
two trapezohedrons are not superposable. It
is said that they are two enantiomorphism shapes. They
are |
| symmetrical
compared to a symmetry plane.) |
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One
speaks about right trapezohedron and left trapezohedron (just as we have a
right hand and a left hand,
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nonsuperposable
thus enantiomorphism).
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| Certain
minerals present sometimes left forms sometimes right forms.
The most known example is that of quartz. |
| The
distinction between left quartz and right quartz has a certain importance
in the electronics industry where one |
| uses
the piezo-electric properties of this mineral. |
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| The
negative crystals of hexagonal shape can contain liquids like water, gas,
as well as rutile needles, tiny hematite |
| discs,
goetite, fushsite or mica. |
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Inclusions of H2O and CO2
in an amethyst from Madagascar. Habitus
type of flattened Dauphiné. |
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This
marvellous Bi-terminated
amethyst of 128 grams also contains water drops in its center. Habitus type of rhomboedron.
 Enlarge
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The
amethyst scepters are also typical in Madagascar. |
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Here,
a scepter in the blackberry shape. |
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There,
the stem of the scepter developed by crystallizing around it a quantity of
small points in hedgehog. Length:40 mm.
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| Amethyst
in castle on its quartz gangue. 986
grams.
Enlarge
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| The
fact that sometimes a thin surface layer of violet color be only presents
in the stone or that this color is not
|
| homogeneous,
makes often a difficult cutting.
The art of lapidary is to place the color correctly in order
to make
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| homogeneous the tone of
cut stones.
The distribution of the color in a cut stone is observed simply
by
|
| immersing
it in water contained in a transparent container and colourless. The color
of amethyst varies from purple
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| reddish
to purple bluish while passing by straightforwardly crimsons tone.
Thus of very variable intensity, the color
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| is
often laid out in stripes parallel to the final faces of the crystal.
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Enlarge
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Your
guide to GGGems |
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©
All of the pictures on this
website have been shot by
gggems.com
|
Alain
Darbellay |
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Text written by Alain Darbellay.
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